Shingled writing is a form of PMR (perpendicular magnetic recording) that is believed to be capable of extending PMR beyond 1 Tb/in2, and has been proposed for future high density magnetic recording by R. Wood et al. in “The Feasibility of Magnetic Recording at 10 Terabits Per Square Inch on Conventional Media”, IEEE Trans. Magn., Vol. 45, pp. 917-923 (2009). In this scheme, tracks are written in a sequential manner from an inner diameter (ID) to an outer diameter (OD), from OD to ID, or from OD and ID towards a middle diameter (MD) in a radial region of a disk in a hard disk drive (HDD). A first track is partially overwritten on one side when a second track adjacent to the first track is written, and subsequently a third track is written that partially overwrites the second track, and so forth. Each track only experiences a one time squeeze from the next track. In shingle writing, each track is only subjected to one neighboring track encroachment while in conventional PMR each track must sustain multiple adjacent track erasures (ATE) from neighboring track writing. Track widths in shingle writing are defined by the squeeze position or amount of overwrite on the next track rather than by the write pole width as is the case in today's hard drives. As a result, write pole width no longer needs to scale with the written track width, and narrow track widths of 60 nm or below can be achieved with a much wider write pole width which is a condition that leads to good writability. In other words, the opportunity for improved writability and higher device yield is not restricted by using pole width as a critical dimension to be tightly controlled.
Referring to FIG. 1, one issue associated with shingled writing is that track edge writing and transition curvature have larger impacts on written tracks compared with conventional writing because of writing from only one corner of the write pole and the one side squeeze associated with each track. This effect is depicted in FIG. 1 where four tracks A-D are sequentially overlaid on one another such that track B partially overwrites track A, track C partially overwrites track B, and track D overwrites a side of track C. Due to the one-side squeeze and the shape of the field contour at one side of the write pole, written transitions will be skewed even when the writer and reader are at zero skew. Note that the transitions shown as alternating light and dark colored bands are all sloped (skewed) with respect to the cross-track direction for tracks A-C that have a one side squeeze.
Secondly, the field gradient at track edge is much lower than at track center. To take advantage of shingle writing, the field gradient at track edge needs to be improved so that write performance is enhanced.
A search of the prior art revealed the following references. In U.S. Pat. No. 7,475,470, a write pole is described with a recessed trailing edge having a curved, triangular, or trapezoidal shape but there is no portion of the trailing edge that is parallel to a leading edge. Therefore, track width is limited to be equal to write pole width.
U.S. Pat. No. 7,477,481 teaches that a write gap thickness is critical but does not mention a recess in a write pole or in a trailing shield. U.S. Patent Application 2005/0073774 discloses a method of reduced side writing.